Relating to watermarks

ABSTRACT

A method and apparatus detects the presence of a watermark in digital data. The digital data may represent picture or sound information and maybe in the form of a broadcast television signal or a signal that has been recorded on a recording medium such as a compact disc. The watermark includes coefficients which have been subject to an inverse local orthogonal transform before being embedded in the input data. In order to detect the presence of the watermark, the input watermarked data is first forward transformed and subtracted from the watermark coefficients so as to derive the data coefficients. The data coefficients are squared and formed into a local average to obtain a measure of the power in the local average. The watermark coefficients are divided by the local average so as to scale them and the scaled watermark coefficients are cross-correlated with the input data to detect whether the watermark is present. The cross-correlation is performed by means of a multiplier receiving the input data as a first input and the scaled watermark coefficients as a second input. The resulting output detection signal is subjected to a thresholding operation using a threshold set in dependence upon the global average power of the input data set.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method and apparatus for detecting awatermark. The invention has particular application to detecting awatermark in a digital picture or sound signal.

2. Description of the Related Art

Watermarking is a well known technique which is used to protect againstthe fraudulent copying or counterfeiting of documents or currency. Thephysical medium carrying the document or currency is marked with adistinctive and recognizable mark which is difficult to remove. In morerecent time a need has arisen to protect digital signals representingpicture information or audio information by means of a distinctivewatermark. A watermark may be used to establish the true origin orownership of the picture or audio information that is represented by thesignals. It can be of particular benefit in the fields of digitallybroadcast television signals and digitally recorded picture or soundsignals.

The watermarking of documents or currency requires the watermark to bereadily visible to assure the person examining the document or currencythat the document or currency is genuine and has not been counterfeited.A physical technique of this sort is of course inapplicable to theprotection of digital signals. Digital signals can be manipulated usingdigital techniques thus removing an existing watermark and possiblyintroducing a fresh watermark. The watermark for protecting a digitalsignal therefore should be obscured from an observer or listener of theinformation represented by the digital signal and should be difficult ifnot impossible to remove. The watermark should still be detectable bythe originator of the watermark to determine if a signal is genuine.

In addition it is advantageous that the watermarking for digital signalsbe sufficiently robust so as to be capable of withstanding thecompression and decompression common in digital broadcasting techniqueswhile still reliably indicating that the signal has been watermarked.

Existing watermarking schemes use a technique in which a watermark isgenerated from a random number sequence and added to the originalpicture or sound information to form a watermarked signal in which thewatermark is obscured. When it is desired to detect the presence of thewatermark, the watermarked signal is correlated with the watermark togenerate a correlation signal which reveals the watermark.

A problem with the existing technique is that the watermark which isrevealed through the correlation may be difficult to perceive. This maybe due in part to the introduction of noise into the watermarked signaland may also be due to the difficulty in distinguishing the watermarkfrom the information represented by the signal.

BRIEF SUMMARY OF THE INVENTION

A need therefore exists to improve the correlation for detecting thewatermark.

According to the present invention there is now provided a method ofdetecting the presence of a watermark in input digital data, thewatermark including coefficients embedded in the data, the methodcomprising the steps of, transforming the data and applying thetransformed data as a first input and the watermark coefficients as asecond input to a cross correlator so as to generate an output detectionsignal, characterised in that the method further includes scaling thecross correlation by a predetermined characteristic of the data.

The invention has the advantage that the output detection signal issignificantly less noisy than in a conventional detector.

Further according to the present invention, there is provided apparatusto detect the presence of a watermark in input digital data, thewatermark including coefficients embedded in the input data, theapparatus comprising means to receive and transform the input data, across correlator having a first input to receive the transformed dataand a second input to receive the watermark coefficients, the crosscorrelator being effective to generate an output detection signal,characterised in that scaling means are provided to scale the crosscorrelation by a predetermined characteristic of the data.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described, by way of example, with referenceto the accompanying drawings in which;

FIG. 1 shows a block diagram of a known apparatus for embedding awatermark into a signal to be protected by the watermark,

FIG. 2 shows a block diagram of a known apparatus for detecting thepresence of a watermark, and

FIG. 3 shows details of apparatus according to the present invention fordetecting the presence of a watermark in a watermarked signal input tothe apparatus,

FIGS. 4 and 5 show a method of generating watermarked coefficients in avideo signals which is subject to a inverse DCT,

FIG. 6 shows a second apparatus according to the present invention fordetecting the presence of a watermark in the watermarked coefficients ofFIGS. 4 and 5,

FIG. 7 shows the location of watermarked coefficients which have beensubject to a forward DCT,

FIGS. 8 and 9 show two images of which one is original and unwatermarkedand the other is watermarked,

FIG. 10 is a graphical diagram of the correlation detected by theapparatus of FIG. 2 including a conventional detector, and

FIG. 11 is a graphical diagram of the correlation detected by theapparatus of FIG. 6.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a source of unwatermarked data 10 which is intended to becombined with a watermark in a combining means 11 to generatewatermarked data 12. The unwatermarked data 10 consists of a stream ofdigital data for example which represents picture or sound informationand may constitute a video signal. The video signal may be a televisionbroadcast signal or may be a video signal which is to be sent forrecording on a recording medium such as a compact disc, laser disc, etc.In an alternative case, the unwatermarked data may be representative ofsound information which is to be broadcast or to be recorded on asuitable recording medium such as a compact disc. The unwatermarked datais divided into a sequence of individual digital data sets or framesbefore being applied to the combining means 11. The watermark is appliedto one or more of the digital data sets or frames. The watermark may bechosen to apply to a given succession of say five data sets or frames ormay alternatively be applied to a known selection of individual datasets or frames.

The watermark consist of a series of watermark coefficients which aregenerated by means of a pseudo random number generator 13. The pseudorandom number generator is a finite state machine which generates a longsecure random sequence of numbers known only to the owner of the data.The pseudo random number generator 13 has an input to receive awatermark modulation key 15 which predetermines the initial state of thenumber generator 13 and therefore the coefficients that are generated bythe number generator 13. The copyright information is embedded by meansof the embedding device 16 into the random number stream from the numbergenerator 13. The copyright information may be a text legend orgraphical information or other.

The copyright information is embedded 13 by means of the embeddingdevice 16 into the random number stream from the number generator 13.The copyright information may be a text legend or graphical informationor other distinguishing information supplied from a copyright source 17.The embedded watermark coefficients are passed to a modulator 18 to besubjected to an inverse local orthogonal transform before being combinedwith the unwatermarked data in the combining means 11. The inversetransform may be chosen from a number of known such transforms includingfor example the inverse discrete cosine transform (DCT) which is wellknown in the field of compression of digital television signals. Thepurpose of the inverse transform is to spread the spectrum of thewatermark. The result is to obscure the watermark when combined with theunwatermarked data.

The inverse transform applied to the watermark coefficients may becontrolled by transform and analysis module 19 using the local dataproperties of the unwatermarked data. The data properties of theunwatermarked data are derived by a forward transform and analysis inthe module 19.

It will be apparent to those skilled in the art that the source ofcopyright information may be omitted so that the watermark coefficientsare applied directly from the number generator 13 to the modulator 18.Furthermore, the module 19 may be omitted so that the inverse transformapplied to the watermark coefficients is not modified by the dataproperties of the unwatermarked data.

The inverse transform employed in the modulator 18 groups the randomnumbers appropriately to create the spectrum that is required. Lowfrequency terms are used in the inverse transformation because thesewithstand compression and decompression more robustly than higherfrequency terms. Turning now to FIG. 2, there is shown a known method ofdetecting a watermark in the data that has been watermarked as describedwith reference to FIG. 1. The watermarked data 12 is applied in theknown detection apparatus as one input to a subtractor 20. Thesubtractor 20 receives the original unwatermarked data 25 as anotherinput and generates an output consisting of the inverse transform of thewatermark coefficients.

Turning now to FIG. 2, there is shown a known method of detecting awatermark in the data that has been watermarked as described withreference to FIG. 1. The watermarked data 12 is applied in the knowndetection apparatus as one input to a subtractor 20. The subtractor 20receives the original unwatermarked data 21 as another input andgenerates an output consisting of the inverse transform of the watermarkcoefficients. A forward transform of the result from the subtractor 20is performed by a forward transform generator 21. In the case where theinverse transform is an DCT, the forward transform is the DCT. Theresult of the result of the forward transform is applied as one input toa detector 22.

The detector 22 has another input from a pseudo random number generator23 which is of the same form and construction as the generator 13 ofFIG. 1. The generator 23 receives a watermark modulation key 24 tocorrespond to the watermark modulation key 14 delivered to the generator13. The detector 22 performs a cross correlation between the forwardtransform produced by the generator 21 with the coefficients generatedby the generator 23 to produce an output detection signal 26. The outputdetection signal 26 may be taken as indicating the presence or absenceof a watermark or may be interpreted to extract embedded copyrightinformation.

Referring now to FIG. 3, a method and apparatus embodying the inventionis shown for detecting the presence of the watermark in the watermarkeddata 12. The watermark data is first transformed by the forwardtransform generator 21 in the manner already described with reference toFIG. 2. The transformed data is applied as one input to a crosscorrelator including a multiplier 30 and a divider 31. The watermarkcoefficients are applied to another input to the detector by means ofthe pseudo random number generator 23. The generator 23 receives thewatermark modulation key 24.

The transformed data coefficients are applied to a squarer 33 to formthe square of the data coefficients from which the local average isformed in an averager 34. The result of the operations in the thesquarer 33 and the averager 34 is to compute a local average 32 of thepower of the data coefficients. These operations may be represented bythe following equation: $\begin{matrix}{{\overset{\_}{F}}_{i}^{2} = {\frac{1}{N_{i}}{\sum\limits_{j\quad \in K_{1}}^{\quad}\quad \left( G_{j} \right)^{2}}}} & (1)\end{matrix}$

where {overscore (F)}_(i) ² is the local average power in K samples ofthe data sequence, K_(i) is the set of neighbours used in thecomputation, N_(i) is the number of members in the set, G_(j) are theset of transformed data coefficients.

The divider 31, divides the watermark coefficients by a scaling factor.This scaling factor is determined by the local average 32 of the powerof the data coefficients. The divider output is applied to themultiplier 30. The multiplier then cross correlates the transformed data21 with the divided watermark coefficients. This produces a detectedcorrelation signal for the set of neighbours used in computing the localaverage power. The detected correlation signal is then applied to aglobal averaging means 35 to form a global average of the correlationsignal for all the samples in the data sequence that makes up the set ofdata 21. The global average is passed to a further divider 36.

The local average computed in the averaging means 34 is applied to aninverter 37 to produce a local average inverse signal. This localaverage inverse signal is applied to a global averager 38 to form theglobal average for the set of data 21 of the inverse signals from theinverter 37.

The global average from the averager 38 is subject to a square rootoperation in the square root means 39 and then applied as a second inputto the divider 36. The division in the divider 36 forms a result whichis a detection signal D which can be represented by the followingequation $\begin{matrix}{{D = {\frac{1}{\sqrt{B}}{\sum\limits_{i\quad \in \quad K}^{\quad}\frac{G_{i}W_{i}}{{\overset{\_}{F}}_{i}^{2}}}}},\quad {B = {\sum\limits_{i\quad \in \quad K}^{\quad}\frac{1}{{\overset{\_}{F}}_{i}^{2}}}}} & (2)\end{matrix}$

The detection signal D is finally subject to a thresholding operation inthe threshold circuit 40 to produce a detector output signal 41.

The detection threshold T is set in the threshold circuit 40 such thatthe probability of the detection signal D being greater than T when thewatermark is not present gives an acceptable maximum false alarmpossibility of P where P is given by the following equation;$\begin{matrix}{P = {\frac{1}{\sqrt{2\quad \pi}}{\int_{T}^{\infty}{\exp \left\{ {- \frac{D_{2}}{2}} \right\} \quad {D}}}}} & (3)\end{matrix}$

It will be apparent from the block diagram of FIG. 3 that the crosscorrelation occurring in the multiplier 30 is performed using thewatermarked coefficients which have been scaled by the scaling factor 32determined by local average. The process of dividing the watermarkcoefficients by the scaling factor 32 reduces considerably the level ofthe noise occluding the estimate of the watermark amplitude.

The scaling factor may be applied to scale the input data instead of thewatermark coefficients. This can be achieved by interposing the divider31 between the transformed data stream 21 and the multiplier 30 insteadof between the random number generator 23 and the multiplier 30. In yetanother alternative, the scaling factor can be sued to scale the resultof the cross correlation from the multiplier 30 instead of the watermarkcoefficients. In general, whichever alternative is chosen for theapplication of the scaling factor the result is to effect a scaling ofthe cross correlation performed by the multiplier 30.

The detection signal D is derived in the divider 36 from the globalaverage supplied by the averager 35 and the square root operationperformed by the square root module 39. The detection signal D has aGaussian distribution when no watermark is present. The Gaussiandistribution is such that the probability that the signal D would exceedthe threshold T can be made extremely remote. A high degree of assuranceis thereby provided of the detection of the watermark when the detectionsignal D exceeds the threshold T.

The apparatus shown in FIG. 3 may be applied to the detection ofwatermarks in video signals as will now be described with reference toFIGS. 4 to 11. The watermark coefficients are generated by first takingempty video frames and dividing each such frame into 16×16 small blocks.The number of blocks in each frame may be of a value other than 16×16 aswill be apparent to those skilled in the art. FIG. 4 shows indiagrammatic form an empty video frame 40 and four of the blocks intowhich it is divided. The first three blocks are numbered 1 to 3 and thelast block is M.

FIG. 5 shows one of the blocks in the frame 40. The block shown in FIG.5 is representative of each of the blocks in the frame 40 and as shownis itself divided into a number of locations. In the specific exampledescribed here, there are 4×4 locations in each block. Some of thelocations in the block are populated with watermark coefficients W₁ toW₅. The chosen locations are distributed in the upper left hand portionof each block.

The inverse DCT of each block is taken to construct the actualwatermark. It will be seen from FIG. 5 that the population of watermarkcoefficients W₁ to W₅ correspond to the low frequency bases of the DCTand that the DC term is excluded. The process of constructing theinverse transform of the watermark coefficients is repeated for all theblocks of a frame so as to construct a complete watermark frame. Thewatermark frame is added to a video signal frame to produce awatermarked video frame. The same process is repeated for all the videoframes which it is desired to mark in a sequence of frames in a videosignal. The apparatus in FIG. 6 is adapted to receive the frames of thevideo signal which have been watermarked as described with reference toFIGS. 4 and 5. The received video signal 60 is transformed by theforward transform generator 61. The forward transform generator devideseach video frame into the same pattern of blocks as before (in thespecific example 16×16 blocks) and takes the forward DCT transformationof each block. The result is to reverse the inverse transformationdescribed with reference to FIGS. 4 and 5.

Following the DCT the watermark image is now represented as a number ofblocks of coefficients. Those coefficients within each block into whichit is known that no watermark was embedded are discarded, leaving thepotentially watermarked DCT coefficients G1 to G5 as shown in FIG. 7.These remaining coefficients are supplied by the forward transformgenerator 61 as one input to the cross correlator comprising amultiplier 60 a and a divider 61 a. The 5 watermarked coefficients ineach block are presented as 6 neighbouring data samples.

The unmarked video coefficients in each block are discarded resulting inthe five marked coefficient locations G₁ to G₅ in each block as shown inFIG. 7. The marked coefficients are supplied by the forward transformgenerator 61 as one input to the cross correlator comprising amultiplier 60 a and a divider 61 a. The 5 marked data coefficients ineach block are presented as 5 neighbouring data samples.

It will be apparent to those skilled in the art that the number of datasamples in each block is a matter of design choice and may be equal to avalue other than 5. In general the number of data samples in each blockpresented to the cross correlator is K samples. The data samplespresented by the forward transform generator 61 takes the form;

{{G₁ ¹ G₂ ¹ . . . G_(K) ¹}, {G₁ ² G₂ ² . . . G_(K) ²}, . . . , {G₁ ^(M)G₂ ^(M) . . . G_(K) ^(M)}}

where G_(i) ^(j) is the ith sample from the block j. The total number Nof the samples is K*M=N.

The watermark coefficients are applied to another input to the detectorof FIG. 6 by means of the pseudo random number generator 23 a whichreceives the watermark key 24 a.

The watermark coefficients W from the generator 23 a are ordered in thesame manner as the transformed data coefficients and appear in the form;

{{W₁ ¹ W₂ ¹ . . . W_(K) ¹}, {W₁ ² W₂ ² . . . W_(K) ²}, . . . , {W₁ ^(M)W₂ ^(M) . . . W_(K) ^(M)}}

The marked data coefficients from the generator 61 are fed into thecross correlator of FIG. 6 in synchronism with the generation of thewatermark coefficients by the generator 23 a. The marked datacoefficients are supplied to a squarer 63 where they are squared beforebeing supplied to an accumulator 64 a. The end of each block of markedcoefficients is detected by a detector 64 b which latches theaccumulator 64 a to cause the accumulated value of the marked datacoefficients to be passed to a multiplier 64 c. The multiplier 64 cmultiplies the accumulated value by 1/N thereby to derive the localaverage 62 of the power of the marked data coefficients in each block.

The watermark coefficients from the generator 23 a pass through a delaydevice 70 to the divider 61 a. The delay device 70 imposes a delay onthe incoming watermark coefficients to compensate for the delay inprocessing the marked data coefficients through the squarer 63, theaccumulator 64 a and the multiplier 64 c. The divider 61 a thus receivesthe local average from the multiplier 64 c in synchronism with thewatermark coefficients from the delay device 70. The divider 61 a scalesthe watermark coefficients of each block by the scaling quantity 62which is the local average power for the marked data coefficients in thesame block. The scaling quantity is clipped by a clip circuit 62C beforeapplication to the divider 61 a. The watermark coefficients are scaled,before application to the multiplier 60 a to enhance the quality of thewatermark amplitude estimate.

It will be apparent that the divider may be interposed between the datastream 61 and the multiplier 60 a instead of between the delay device 70and the multiplier 60 a. In this case the scaling quantity 62 would beemployed to scale the data stream 61 instead of the watermarkcoefficients. In yet another alternative, the scaling quantity 62 may beused to scale the output from the multiplier 60 a.

The local average for each block generated by the multiplier 64 c isapplied to an inverter 67 to produce a local average inverse signal. Thelocal average inverse signal from the inverter 67 is applied to anaccumulator 68 a which accumulates successive inverse local averages.The end of each video frame signal is detected by an end of framedetector 68 b which supplies an end of frame signal to the accumulator68 a. The end of frame signal latches the accumulator 68 a to pass theaccumulated inverse local average values therein to a multiplier 68 c.The multiplier multiplies the output from the accumulator 68 a by 1/Nthereby to derive a global average over each frame of the power of themarked data coefficients. A square root module 69 generates the squareroot of the global average from the multiplier 68.

The cross correlation performed by the multiplier 60 a results in across correlation signal representing the correlation between thewatermark coefficients and the marked data coefficients for each blockin each frame of the video signal data. The cross correlation signalsare passed to an accumulator 65 a where the cross correlation signalsare accumulated. The end of frame signal from the detector 68 b latchesthe accumulator 65 a and causes the accumulator to pass the accumulatedcross correlation signals to a multiplier 65 b. The multipliermultiplies the accumulated cross correlation signals by 1/N to derive aglobal average for each video frame.

The global average of the cross correlation signals is divided by theoutput from the square root module 69 to produce an output detectionsignal D. The output detection signal D is subject to a thresholdingoperation by a threshold circuit 71 to produce a detector output signal72.

Referring now to FIGS. 8 and 9 there are shown two samples of a videoimage. FIG. 8 shows an original image which includes no watermarkedcoefficients. FIG. 9 shows the same image but modified to includewatermarked coefficients. Whilst the two images resemble one another soclosely that the watermark is obscured to a viewer, it is necessary thatthe detector of the watermark should be able to discriminate between thetwo images with a high degree of assurance that one contains a knownwatermark whilst the other does not.

FIG. 10 shows the response to be expected from a conventional detector.The detector output line 81 shows the detected correlation between thecorrect watermark coefficients and a watermarked video image such asthat of FIG. 9. The detector output line 82 shows the detectedcorrelation between a set of incorrect watermark coefficients and thewatermarked image. Two possible threshold levels 83 and 84 are shown inFIG. 10 and it will be seen that the discrimination between a correctwatermark and an incorrect watermark is relatively narrow. A thresholdset at the level 84 would be unsatisfactory because there is thepossibility of falsely detecting a watermark as shown where the line 82exceeds the threshold 84. The threshold 83 is satisfactory but must beset carefully to avoid the possibility that the detector output line 81does not drop below the threshold.

It is to be understood that modifications could be made and that allsuch modifications falling within the spirit and scope of the appendedclaims are intended to be included in the present invention.

A feature of the detector output lines 81 and 82 in FIG. 10 is that bothdeviate substantially from a median line. This is because of therelatively high noise levels associated with such a detector.

Turning now to FIG. 11, there is shown the response from the detector ofFIG. 6. The line 91 shows the detected correlation between the correctwatermark coefficients and the marked data coefficients of a video imagesuch as shown in FIG. 9. The line 92 shows the detected correlationbetween incorrect watermark coefficients and the video image. Twopossible threshold levels are shown at lines 93 and 94. It will beobserved that the detector outputs represented by the lines 91 and 92show a wide discrimination between a correct and an incorrect watermark.The output in each case is also subject to much less deviation than thecorresponding output shown in FIG. 10. As a consequence the thresholdfor the detection of a correct watermark can be set at a value whichsubstantially removes uncertainty in detecting the watermark.

What is claimed is:
 1. A method of detecting the presence of a watermarkin digital data, the watermark including coefficients embedded in thedata, the method comprising the steps of, spectrally transforming thedata and applying the transformed data as a first input and thewatermark coefficients as a second input to a cross correlator toprovide a cross correlation, scaling the cross correlation by a localaverage characteristic of the data to form a detected correlationsignal, forming a global average of said correlation signal, applyingsaid global average of said correlation signal to one input ofarithmetic means, forming a global average of the transformed data andapplying the resultant to another input of said arithmetic means to forma detection signal.
 2. A method as claimed in claim 1, wherein the stepof scaling comprises scaling the watermark coefficients applied as thesecond input to the cross correlator.
 3. A method as claimed in claim 1,wherein the step of scaling comprises scaling the transformed dataapplied as the first input to the cross correlator.
 4. A method asclaimed in claim 1, wherein the transformed data is applied to squaringmeans and averaged to form said local average characteristic.
 5. Amethod as claimed in claim 1, wherein the scaling is performed bydividing the watermark coefficients by the local average characteristic.6. A method as claimed in claim 1, wherein the detection signal issubjected to a thresholding operation.
 7. A method as claimed in claim1, wherein said arithmetic means comprises a square rooter and adivider.
 8. Apparatus to detect the presence of a watermark in inputdigital data, the watermark including coefficients embedded in the inputdata, the apparatus comprising means to receive and spectrally transformthe input data, a cross correlator having a first input to receive thetransformed data and a second input to receive the watermarkcoefficients so as to form a cross-correlation signal, saidcross-correlator including scaling means for scaling the transformeddata by a local average characteristic of the data to form a detectedcorrelation signal, means for forming a global average signal of saidcorrelation signal and applying said global average signal to one inputof arithmetic means, means for forming a global average of thetransformed data and applying the resultant to another input of saidarithmetic means whereby the output of the arithmetic means is adetection signal.
 9. Apparatus as claimed in claim 8, wherein thescaling means is adapted to scale the cross correlation by scaling thewatermark coefficients applied to the second input to the crosscorrelator.
 10. Apparatus as claimed in 8, wherein the scaling means isadapted to scale the cross correlation by scaling the input data appliedto the first input to the cross correlator.
 11. Apparatus as claimed inclaim 8, further comprising a squarer to receive the transformed datawhich is averaged to form said local average characteristic. 12.Apparatus as claimed in claim 8, wherein the scaling means comprise ascaling divider to divide the watermark coefficients by the localaverage characteristic.
 13. Apparatus as claimed in claim 8, wherein thecross correlator comprises a multiplier to multiply the scaled watermarkcoefficients with the transformed input data.
 14. Apparatus as claimedin claim 8, further comprising thresholding means to subject thedetection signal to a threshold.
 15. Apparatus as claimed in claim 8,wherein the arithmetic means comprises a square rooter and a divider.16. Apparatus to detect the presence of a watermark includingcoefficients embedded in digital input data, said apparatus including:means to receive and spectrally transform the input data to thereby formtransformed data; local averaging means for providing a local average ofthe power of the transformed data at first and second outputs thereof;cross-correlation and scaling means connected to the first output of thelocal average means to scale the coefficients embedded in the input dataand to cross-correlate the transformed data with the scaled coefficientsto produce a correlation signal; first global averaging means to form aglobal average of the correlation signal; local average inverse meansconnected to the second output of the local average means so as toprovide a local average inverse signal; second global averaging means toform a global average of the local average inverse means; and arithmeticmeans to receive the global average of the correlation signal and thelocal average inverse signal so as to form a detection signal. 17.Apparatus as claimed in claim 16 wherein the transformed data is appliedto a squarer connected to an input of the local average means. 18.Apparatus as claimed in claim 16 wherein the cross-correlation andscaling means includes a divider to divide the coefficients by the localaverage of the power of the transformed data.
 19. Apparatus as claimedin claim 18 wherein the cross-correlation and scaling means furtherinclude a multiplier to multiply the scaled coefficients with thetransformed data.
 20. Apparatus as claimed in claim 16 wherein thearithmetic means comprises a square rooter and a divider.
 21. Apparatusas claimed in claim 16 further comprising thresholding means to subjectthe detection signal to a threshold.
 22. A method of detecting thepresence of a watermark in digital data, said watermark includingcoefficients embedded in the data, the method comprising the steps of:spectrally transforming the input data to form transformed data;providing a local average of the power of the transformed data at firstand second outputs of a local average means; scaling the coefficientsembedded in the data from the first output of the local average meansand cross-correlating the transformed data with the scaled coefficientsto thereby produce a correlation signal; providing a local averageinverse signal of the second output of the local average means; forminga global average of the local average inverse signal; receiving theglobal average of the correlation signal and the local average inversesignal to form a detection signal.